1. Field of the Invention
This invention relates generally to mechanical-to-electrical sensing structures, and more particularly to mechanical-to-electrical sensing structures that use electrochemistry to define gage links with P-type piezoresistor elements.
2. Description of the Related Art
In electromechanical transducers a transducing element is utilized for detecting the relative displacement of two parts and for developing a corresponding electric signal. Generally, such relative displacements have been measured in the past with various kinds of strain gages. However, these have a tendency to be of considerable weight, some of which are very bulky, some of which are not very sensitive. Those that are have intricate designs which are very expensive.
Force-type sensors or gages are known which are mounted between two parts between which a force is applied. The gage is, therefore, strained in an amount which depends upon that force.
As piezoresistive transducers have developed in use over the years, it has become increasingly desirable to have extremely small sensors of high sensitivity and low bulk. However, in order to develop force gages which are of extremely small size, difficulties arise in the handling thereof for subsequent mounting upon their substrate, once they are developed. They are difficult to handle not only because of their small size, but also because of their fragility.
One of the primary advantages of force transducers lies in the fact that the displacement between the pads at each end thereof produced by relative motion of the two parts to which the pads are attached is concentrated in the “suspended”, so to speak, portion of the force gage which can mechanically amplify the strain being sensed or measured. Furthermore, the resistance change of the element per unit displacement is greatest as the length of the element is reduced. By use of both short gage lengths and appropriate leverage very large resistance changes may result from very small displacements. This change in resistance is determined by means of electrical current flowing through the element from one pad to the other, and measuring changes in voltage or other electrical properties resulting from changes in resistance. However, when attempts are made to reduce to a smaller size such force gages, then, as mentioned above, difficulties arise relative to the handling thereof in mounting upon their substrates, as well as other problems which ordinarily arise in handling very small objects.
Strain sensitive elements are provided in the form of force gages which are derived from the substrate upon which they are subsequently supported in use. That is, the gages are defined upon the substrate or marked thereon, and subsequently etched right from the material of the substrate. In one form of force gage, the gage is etched to allow a small support or mesa underneath, while maintaining the gage still connected by this minute portion of the substrate to the substrate proper. In its preferred form, the invention is directed to a force gage which is etched free of its substrate along its length but continuous with it at its ends. Thus, the gages of the invention are crystallinally continuous with their support.
That is, force gages of substantially smaller strain volume are produced by defining the gage in the substrate or in material rigidly bonded to the substrate, and subsequently etching away the immediately adjacent material, leaving the gage free in space, after the fashion of force gages of the past, but supported against unwanted cross loads by remote portions of the substrate. Such gages may have volume as small as 3×10−10 cubic centimeters of stressed material, as opposed to present commercially available force gages wherein the strained volume is 5.×10−7 cubic centimeters. Both gages would typically be strained to one part per thousand. The strain energy is thus a thousand fold less for the smaller gage.
Gages on this type typically have dimensions of about 0.6E-4 cm×4E-4 cm×12E-4 cm, 3E-11 cubic cm, 50 ohms. In one force gage, a conventional silicon crystal material is selected, and the outline of the gage is etched on the selected crystal which forms the substrate. An etch is selected which is both anisotropic and doping-selective. Caustic, hydrazine, and pyrocatechol etchants may be selected, depending upon the results desired. They attack silicon rapidly in the [112] direction, moderately rapidly in the [110] direction, and very slowly in the [111] direction. With this invention, the substrate orientation is (110) plane and [111] along the gage so as to define a groove over which the gage extends. With such orientation, a groove is produced with walls which are nearly vertical, and with floors that are nearly flat.
The same etchants which are anisotropic are dopant selective, in that they attack very slowly silicon in which a boron concentration is developed which is greater than 5×1019/cc. In accordance with the process of the invention, the gage is defined and its terminals are also defined by a planar diffusion or ion implantation through an oxide mask to a boron concentration of roughly 10×20/cc. The boron makes the gage P-type, while the substrate is N-type. The diffused area is electrically isolated from the substrate by a P-N junction. During the etching procedure which forms the groove, the gage is exposed to the etchant, but is resistant to it. As will be appreciated, and explained further herein, when the groove is defined over which the gage extends, a hinge is also defined in the substrate around which one end of the substrate moves relative to the other to develop the strain being monitored by the sensor. Also, the hinge protects the gage against transverse loads. Not all of the anisotropic etchants are also doping selective.
It is noted that the gage material spared by the dopant-selective etch is necessarily highly doped and therefore of low resistively, typically 0.001 ohm-cm. This makes the individual gages have resistance which is inconveniently low for conventional circuitry. For example, a “sturdy” gage would have resistance only 13 ohms, and a smallest old-art gage 50 ohms. Free-standing transducers for the general market are expected to have resistance well over 100 ohms and 1000 ohms is desired. It is necessary, therefore, to set several of these gages electrically in series, mechanically in parallel, to achieved an acceptable resistance. Each added gage needs the same strain energy from the mechanical signal source, so the system sensitivity declines in order to bring its resistance up. In another force gage, two substrate wafers are bonded together. Grooves are formed either before or after bonding of the wafers, gages and their terminals are defined in the gage wafer by doping them to the requisite high concentration of boron before bonding the wafers, then etching away all of the undoped portion of the gage wafer. Alternatively, the whole bonded surface of the gate wafer is doped with boron so that the etching leaves a continuous sheet of gage material from which gages may be etched by a subsequent photolithographic step.
Once the two wafers are bonded together, with the gages positioned over their appropriate grooves or apertures which have been defined in the wafers, then the gages are freed by etching away all of the gage wafer except the gages and their terminals.